The present invention relates generally to sensors and, more particularly, to a method of canceling quadrature error in a dynamically decoupled angular rate sensor.
This invention involves a class of sensors that use a vibratory element for measuring angular velocity. These sensors (including others of different construction) are commonly referred to as gyros (for gyroscopes), or in the case or very small gyros, micro-gyros. In micro-gyros, the elements are small, typically around 1 square millimeter.
Micro-gyros are generally produced from silicon wafers, using photolithographic techniques, in accordance with the principles of Micro-Electro-Mechanical Systems (MEMS). The small size of these elements is necessary to enable the production of large numbers of micro-gyros from a single silicon wafer using micro-fabrication techniques.
A micro-gyro measures the angular rate of rotation about an input axis or so-called xe2x80x9crate axisxe2x80x9d. Micro-gyros may generally be classified as linear or as rotary. In either case, a mass is driven into vibration relative to a xe2x80x9cdrive axisxe2x80x9d that is orthogonal to the rate axis. An electrostatic comb-drive structure is commonly used to oscillate the mass.
In a xe2x80x9clinearxe2x80x9d micro-gyro, the mass is driven to vibrate along the drive axis. In a xe2x80x9crotaryxe2x80x9d micro-gyro, the mass is driven to vibrate about the drive axis. In either case, if the mass is subject to rotation about the sensor""s rate axis at some angular rate of rotation, then coriolis forces acting on the vibrating mass will naturally cause it to also vibrate along or about a xe2x80x9csense axisxe2x80x9d that is orthogonal to the rate and drive axes.
Some micro-gyro embodiments have only a single xe2x80x9cproof massxe2x80x9d that is both driven and sensed. U.S. Pat. No. 5,992,233 entitled xe2x80x9cMICROMACHINED Z-AXIS VIBRATORY RATE GYROSCOPExe2x80x9d is representative of a linear micro-gyro wherein a single xe2x80x9cproof massxe2x80x9d is driven into vibration along a drive axis and wherein coriolis-induced motions of that same proof mass are detected along a sense axis that is orthogonal to the drive axis. The proof mass, in other words, is both the drive mass and the sense mass.
Other micro-gyros provide a drive mass that quite literally carries a sense mass. The sense mass is coupled to and moves with the drive mass as both vibrate along the drive axis, but the sense mass is free to move along the sense axis under the presence of coriolis forces.
The unique micro-gyros developed by the assignee of this invention are statically xe2x80x9cde-coupledxe2x80x9d in that the drive mass and sense mass may move independently of one another. In the absence of rotation rate, and under ideal conditions, the driven mass is vibrated but the sense mass remains still. In the presence of rotational rate, however, coriolis-induced energy is dynamically transferred from the drive mass (vibratory element) to the sense mass through suitably designed flexures. The operational concepts of a decoupled micro-gyro design are disclosed in U.S. Pat. No. 5,955,668, commonly owned by the assignee of this invention and hereby incorporated by reference in its entirety.
A major challenge in designing the above-described micro-gyros is dealing with manufacturing imperfections and expected variations due to normal manufacturing tolerances. In operation, as explained above, the vibratory element is driven to oscillate along or about the drive axis. When the vibrated element is subject to an angular velocity about the rate axis, the vibrated element responds by exhibiting a small vibration along or about the third direction, or sense axis. In the ideal micro-gyro, the input, rate and sense axes are mutually orthogonal.
A major source of error in micro-gyros is xe2x80x9cquadrature error,xe2x80x9d a condition that relates to the erroneous coupling of drive motion into sense motion in the absence of a rotational rate. This coupling is caused by imperfections in the manufacturing process. More particularly, the coupling will occur whenever the support structures that cause the vibrating element or elements to move along the input and output direction are not perfectly orthogonal. The output signal induced by such drive error is usually referred to as the quadrature signal. The output signal or sense signal in an imperfect micro-gyro, therefore, contains both the desirable rate signal and the undesirable quadrature signal.
FIG. 1 is a simplified diagram of a linear micro-gyro wherein the vibratory element consists of a single mass 10. The present invention is most easily applied to a dynamically decoupled micro-gyro, but it is helpful to start with an explanation of a single-mass gyro like this one in order to understand quadrature error. Here, when the mass 10 is vibrated along the perfect drive path 21, then the mass 10 will respond to rotation about a rate axis that is perpendicular to the paper by exhibiting a small degree of vibration along the sense path 31. If manufacturing imperfections cause the mass 10 to vibrate along the imperfect drive path 22, rather than the perfect path 21, then the drive has a quadrature error component 23 that is parallel to the sense path 31. The quadrature component 23 of the imperfect drive 22 is nominally detected as sense vibration.
Quadrature error is troublesome because the error signal can be very much larger than the sense signal induced by coriolis forces. Because of this, the industry has undertaken considerable effort to eliminate and/or compensate for quadrature error.
One common method used to remove the quadrature error is known as xe2x80x9csynchronous demodulation.xe2x80x9d It relies on the fact that the rate signal is 90 degrees out-of-phase relative to the quadrature signal, meaning that the rate signal is in-phase with the drive signal. It operates by multiplying the output containing both rate and quadrature with the drive signal, and then passing the resultant signal through a low pass filter. This method works with limited effectiveness because the quadrature is typically 10,000 to 100,000 times larger than the rate at the low range of operation. The relatively large magnitude of quadrature means that error due to phase needs to be tightly controlled, typically to 0.01 degree or lower. This stringent phase control must also be maintained over the full operating temperature range.
The ""233 patent first discussed above offers another method of quadrature error correction. In particular, it reveals a technique for active correction of quadrature by applying an oscillatory force that counteracts the inaccuracy of movement as the mass moves along the drive axis. The technique disclosed in the ""233 patent, however, has several major shortcomings:
First, it can only be used with linear micro-gyros having a mass that vibrates along an axis and not with rotary micro-gyros that have a mass vibrates about an axis.
Second, in order to avoid affecting the coriolis signal, the correction forces must be applied in exact magnitude proportional to the position of the element along the path. The further away is the element from its neutral position, the higher the correction is required.
Third, the same electrodes used for reducing quadrature are also used for affecting the frequency of the element about the sense axis, resulting in a compromise between reducing quadrature or frequency mismatch. The same electrodes are also used for sensing the deflection of the mass element.
Fourth, the oscillatory input may have a parasitic capacitance coupling into electrodes used to measure sense output.
The known methods of correcting for quadrature error need improvement and, even if improved, the known methods are not well-suited for use in a decoupled micro-gyro. There remains a need, therefore, for an improved method of canceling quadrature error in an angular rate sensor and, more particularly, for a method of correcting quadrature error that is suitable for use in a decoupled micro-gyro.
The invention resides in a method of correcting quadrature error in a dynamically decoupled micro-gyro having a drive mass that is vibrated relative to a drive axis and a sense mass that responds to the drive mass in the presence of an angular rate about a rate axis and a corresponding coriolis force by vibrating relative to a sense axis, the method comprising the steps of: providing a first static force element for applying a first steady-state force to a first region of the drive mass; providing a second static force element for applying a second steady-state force to a second region of the drive mass; and applying a corrective steady-state force to the drive mass with the first and second static force elements, the corrective steady-state force re-orienting the drive mass to make the drive axis of the drive mass orthogonal to the sense axis of the sense mass. The corrective steady-state force preferably re-orients the drive mass by repositioning the drive mass about the rate axis. The static force elements may generate the necessary forces in any suitable manner, but the preferred static force elements comprise first and second electrodes that provide an electrostatic force.
In the preferred embodiment, the method comprises the further steps of: connecting the drive mass to a ground voltage; holding one of the first and second electrodes at the ground voltage; and setting the other of the first and second electrodes to a voltage that is different than the ground voltage such that a corrective steady-state force of suitable direction and magnitude is applied to the drive mass.